A radically different mechanism for S-adenosylmethionine-dependent methyltransferases

Area of Science:

• Biochemistry
• Molecular Biology
• Enzymology

Background:

• Methylation is vital for cellular processes, often mediated by S-adenosylmethionine (SAM)-dependent methyltransferases using an S(N)2 mechanism.
• Radical SAM enzymes represent a distinct class, capable of methylating non-nucleophilic carbon atoms via a 5'-deoxyadenosyl 5'-radical intermediate.

Purpose of the Study:

• To elucidate the reaction mechanisms of two radical SAM enzymes, RlmN and Cfr.
• To understand how these enzymes catalyze methylation at the sp(2)-hybridized carbons of adenosine 2503 in 23S ribosomal RNA.

Main Methods:

• Mechanistic studies of RlmN and Cfr enzymes.
• Investigation of radical SAM enzyme activity on ribosomal RNA substrates.

Main Results:

• The methylation of adenosine 2503 by RlmN and Cfr does not involve direct methyl transfer from SAM to the RNA.
• Both enzymes utilize a ping-pong mechanism, featuring an intermediate methylation of a conserved cysteine residue.

Conclusions:

• Radical SAM enzymes employ a novel methylation strategy distinct from canonical S(N)2 methyltransferases.
• The identified ping-pong mechanism provides new insights into the functional diversity of radical SAM enzymes in biological systems.